COMPENSATION FOR RADIAL DISPLACEMENT 
All points above a plane passing through the lowest elevation as represented 
on a vertical aerial photograph, are displaced radially from the center of the photo- 
graph. The amount of this displacement varles with the altitude of the points above 
the lowest point of elevation, their distance from the center of the photograph, and 
the focal length of the taking camera. This displacement causes a change in the angle 
of slope seen stereoscopically on a pair of vertical aerial photographs. The amount of 
this apparent change of the angle of slope will depend on the location and direction 
of the slope in the stereoscopic model. The apparent inclination of slopes known to be 
similar will appear different at different locations and orientations within the stereo- 
scopic model. 
The targets of the Stereo-slope Comparator are displaced in a similar fashion, 
but from a perspective point at the eye pieces of the stereoscope. If the photographs 
are properly alined beneath the stereoscope, displacement of the targets in a large 
part compensates for the displacement of the photographic images. 
APPLICATION AND ACCURACY 
Measuring angles of slope in the stereoscopic model using the Stereo-slope 
Comparator has two excellent applications in photointerpretation work: a means of 
rapidly measuring angles of slope in the stereoscopic model, which is essential In many 
flelds of photointerpretation work such as photogeology and engineering studies, and 
the training of new personnel in recognizing the existence of vertical exaggeration 
and understanding the factors that cause it. 
A complete evaluation of the Stereo-slope Comparator method of measuring 
angles of slope is in progress. To date the results are favorable. Several hundred 
determinations of true angles of slope were made on a constructed stereoscopic model. 
The model contained 108 angles of slope ranging from 2 degrees to 90 degrees. The 
slopes were at random locations and directions. Measurements of the exaggerated 
angles of these slopes and their conversion to true angles of slope gave the following 
results: 2 to 4 degree angles of true slope were read with a maximum error of plus 
or minus | degree; 20 to 25 degree angles of true slope, with a maximum error of plus 
or minus 3 degrees; 40 to 50 degree angles of true slope, plus or minus 7 degrees; and 
80 to 90 degree angles of true slope, plus or minus [0 degrees. 
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